Development of technologies for utilisation of plant biomass to replace petroleum as a precursor for fuels and chemical products has recently become commercially attractive. In fact, lignocellulosic biomass processing has become a central focus in ongoing efforts to develop sustainable economies and ameliorate global climate change arising from greenhouse gas emissions.
Utilisation of lignocellulosic biomass in liquid fuels has attracted particular attention. A variety of schemes for this have been proposed. In some systems, whole plant biomass is subject to pyrolysis at temperatures between 500 and 700° C. to produce a combustible bio-oil comprising degradation products derived from lignin, cellulose and hemicellulose. See e.g. Lehto et al. 2014. In other systems, whole plant biomass is extracted in solvents such as water, alcohols or water-alcohol mixtures, typically at supercritical temperature and pressure conditions. See e.g. Aysu et al. 2012; Huang et al. 2013. The extracted, degraded material similarly comprises a combustible liquid fraction. In still other systems, plant biomass is processed using a so-called “biorefinery” approach, which seeks to optimise commercial utilisation of the various different chemical components of the feedstock. The carbohydrate polymers cellulose and hemicellulose are transformed into monomeric sugars which can then be fermented to a variety of different end products such as fuel ethanol or fatty precursors of bio jet-fuel. The lignin component is then recovered as a residual or “waste” product. In cellulosic ethanol biorefineries, which are already commercially viable, fermentable sugars are produced by enzymatic hydrolysis of cellulose and hemicellulose and then fermented to ethanol, which can be used directly as liquid fuel or blended into gasoline. The lignin component is then recovered as a residual, either from enzymatic hydrolysate, from fermentation broth before or directly after distillation of ethanol, or after anaerobic digestion of distillery vinasse.
Lignocellulosic biomass such as agricultural wastes and so-called “fuel grasses” comprise a significant percentage of lignin, typically between 15-35% by weight. Native lignin, which is intricately associated with cellulose and hemicellulose strands, is a complex, hydrophobic, branched, highly cross-linked, amorphous biopolymer formed by oxidative coupling of “lignol” phenyl-propanoid monomers—p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol. Lignin also typically comprises covalently attached carbohydrate moieties. The number-average molecular weight of native lignins typically falls within the range 7500-15000. In cellulosic ethanol biorefineries, the biomass feedstock is typically subject to some form of pretreatment in order to improve accessibility of cellulose strands to productive enzyme binding. Pretreatment typically involves heating to temperatures above 150° C. and causes a partial de-polymerisation of lignin such that the number-average molecular weight is reduced to between about 1500 and 8000.
Commercial viability of a cellulosic ethanol biorefinery can turn critically upon the economic value obtained from residual lignin. Simply drying residual lignin and using it as a solid fuel provides a comparatively low value lignin product. Higher value can be obtained where residual lignin is converted into a liquid fuel, particularly where the liquid fuel can be blended into diesel fuels.
Lignin fuels are especially valuable when they are suitable for blending with marine diesel, which comprises a greater percentage of heavy oils. Many countries have imposed restrictions on permissible sulphur remissions from marine transport. Ordinary, low-grade fuel oils typically used in container shipping have high sulphur content, 2% by weight or more. Demand is accordingly increasing for higher quality, low-sulphur marine diesel fuels. Lignin residual from a cellulosic ethanol biorefinery can normally be obtained having very low sulphur content. Liquid fuel derived from such lignin residual can be advantageously used as a low-sulphur blend in marine diesel, analogous to use of fuel ethanol as a blend in gasoline.
Biomass pyrolysis oils and crude liquid fuels derived from lignin can be used as diesel blend fuels through use of emulsifiers, see e.g. Martin et al. 2014, or in some cases co-solvents such as alcohols or tetrahydrofuran, typically used in large quantity see e.g. Alcala and Bridgwater 2013; Yaginuma et al. 2001; WO2013/009419. However, reliance on emulsifiers and co-solvents increases process cost and complexity, especially when used in large quantity. It is commercially advantageous to produce a lignin-derived liquid fuel that is, itself, directly soluble in diesel, and particularly in marine diesel. But this has proved difficult to achieve, especially with lignin residual from a cellulosic ethanol biorefinery. This material is typically less than 75% pure, further comprising salts, residual carbohydrate and so-called “pseudo-lignin” (chemical by-products of pretreatment processes).
De-polymerisation at high temperatures is generally adequate to convert solid residual lignin into a combustible liquid. But in order for lignin-derived liquid fuels to be diesel-soluble, the oxygen content of the product oil in hydroxyl, ether, ketone and aldehyde groups should be substantially reduced compared with the lignin starting material. Reduction of oxygen content is associated with an increase in hydrogen content. Native lignin residual from a biomass refinery process typically has a molecular O:C (oxygen:carbon) ratio of between 0.30 and 0.75 and an H:C ratio of between 1.1 and 1.3. In order for lignin-derived oils to become substantially soluble in marine diesel, the O:C ratio of the product oil should generally be reduced to levels of 0.20 or less. To achieve such low levels, the O:C ratio of the lignin starting material should typically be reduced by at least 50%, and the H:C ratio should typically be increased to greater than 1.5.
In order to achieve substantial de-oxygenation of lignin-derived liquid fuels, it has previously been considered necessary to employ a commercially disadvantageous separate de-oxygenation step or a reaction promoter added during depolymerisation. See e.g. WO2010/094697 [formic acid reaction promoter added to de-polymerisation reaction with lignin residual]; US2012010318 [separate catalytic hydrotreatment step for reduction of oxygen content in heavy pyrolysis bio-oil]; WO2013/135973 [separate multi-stage pressure and heat treatment for deoxygenation of pyrolysis bio-oil]; US20110119994 [separate catalytic hydrotreatment of pyrolysis bio-oil]; U.S. Pat. No. 7,578,927 [separate catalytic hydrotreatment of pyrolysis bio-oil].
One approach to de-polymerisation is solvolysis of residual lignin in supercritical alcohol or alcohol-water mixtures. For review, s Wang et al. 2013. However this approach has been reported previously only using heavily processed and comparatively pure lignin material such as Kraft lignin from pulp and paper processing, which has been solubilised and partially derivatised, or lignin that was previously extracted using alkali or “organosolv” processes. Furthermore, in prior art processes, substantial reductions in O:C ratio could be obtained using supercritical ethanol solvolysis of lignin material only where reaction promoters such as H2 gas, acids, bases, hydrogen donor co-solvents such as formic acid and/or catalysts such as metal oxides or formic acid were added to the reaction. See e.g. Guvenatam et al. 2016 [alkali lignin w/catalyst]; Kuznetsov et al. 2015 [alkali lignin w/catalyst]; Huang et al. 2014 [alkali lignin w/catalyst]; Kim et al. 2013 [organosolv lignin w/H2 gas]; Cheng et al. 2012 [Kraft lignin w/H2 gas]; Ye et al. 2012 [organosolv lignin from enzymatically hydrolysed corn stover w/H2 gas]; Kleinert et al. 2011 [alkali and organosolv lignin w/formic acid]; Kleinert and Barth 2008 [alkali lignin w/formic acid].
Lignin material that has been previously extracted is typically chemically altered, substantially de-polymerized, and more readily soluble in alcohol solvents. In contrast, residual lignin from a cellulosic ethanol biorefinery process is less chemically altered, less readily soluble and comparatively impure. We have discovered that this comparatively impure solid lignin residual can be directly converted into a significantly diesel-soluble liquid fuel using solvolysis in supercritical ethanol, without requirement for any pre-extraction of lignin content or for any added reaction promoter. By selecting appropriate lignin:solvent ratio, water content, reaction temperature and reaction period for the supercritical alcohol treatment, product oils can be obtained using such a process that have O:C ratios of 0.20 or less. Very surprisingly, supercritical alcohol treatment of either crude lignin residual or refined pre-extracted lignin can be advantageously conducted at much higher loading of lignin in solvent than has previously been contemplated, thereby minimizing solvent consumption.